The invention concerns a nozzle for delivering low temperature coolant to a position within a room-temperature atmosphere with improved flow characteristics.
X-ray crystallography is typically performed by diffracting x-rays through a crystalline sample and determining the resultant pattern of diffracted radiation on a detector or target. In many applications, the sample must be frozen prior to and during testing to maintain the crystal structure of the sample. In such cases, the need to maintain the crystal in a frozen state is complicated by the continual influx of x-ray energy, part of which is absorbed by the sample. As the sample absorbs energy, it heats, and such heating must be offset by a cooling mechanism if the sample is to be retained in a frozen state. Further, because additional testing of samples after an initial x-ray diffraction test is often desirable, it is often necessary to be able to insure the frozen state, and thereby the integrity, of a sample throughout the x-ray crystallography process and afterward.
One of the mechanisms for maintaining a sample in a frozen state is to provide a steady coolant stream to the sample""s location. Very cold nitrogen gas is often used as the coolant stream because nitrogen is readily available, techniques for refrigerating it are well known, and it does not introduce environmental hazards into the working area. However, a variety of other gases, including helium and other noble gases, can readily be used to provide such a coolant stream.
The x-ray crystallography process is normally carried out inside of an ambient, e.g. room-temperature, environment, so that when crystals undergoing crystallography must be maintained at a specialized temperature, such as approximately xe2x88x92180xc2x0 C., a specially controlled temperature zone must be created. Further, the physical requirements of the x-ray crystallography process require open space around the sample, so that the apparatus directing the coolant stream cannot be placed in immediate proximity to the sample. Generally, the coolant stream which creates the special temperature zone for the crystals is directed by a nozzle which is preferably spaced apart from the sample so that the nozzle does not interfere with the x-rays incident to the sample or diffracted therefrom, and so that the nozzle does not interfere with the necessary movements of other apparatus. The nozzle generally comprises a vacuum jacket to insulate the coolant stream from the ambient atmosphere until the coolant stream exits the nozzle.
To provide maximum cooling of the sample and to avoid icing on the sample, it is desirable that the coolant stream be dry and that it flow smoothly. However, the physical circumstances described above hinder these goals. Even if the coolant stream leaves the nozzle in a laminar flow state, the outer zone of the coolant stream is rapidly heated by contact with the much warmer ambient atmosphere. As it heats, the coolant stream gas expands, and may do so unevenly, introducing turbulence into the coolant stream flow. The flow may be further disrupted by normal air currents in the room. Further, even if the coolant stream gas is dry when it exits the nozzle, the induced turbulence and mixing with moisture in the room air may create icing problems at the sample.
The greater the distance between the nozzle and the sample, the more time these effects have to disrupt the flow of the coolant stream. Thus, even though it is desirable to move the nozzle out of the way of the x-ray crystallography equipment and the x-rays themselves, it is sometimes impossible to achieve these goals, and the testing equipment must instead be adjusted to accommodate the cold stream nozzle.
To offset the disruption of the coolant stream caused by this rapid heating on contact with the ambient atmosphere, it is possible to pre-warm the outer zone of the coolant stream just after it exits the nozzle. Generally, this pre-warming is accomplished by fitting a hollow cylindrical heating element to the exit port of the nozzle. The internal diameter of the heating element is essentially the same diameter as the exit port, and thus of the coolant stream. By controlling the electrical current in the heating element, the outer zone of the coolant stream can be heated to a desired temperature as the coolant stream passes through the heating element. Generally, the outer zone of the coolant stream will be warmed to a temperature intermediate that of the ambient atmosphere and the inner zone of the coolant stream. Thus, the warmed outer zone of the coolant stream provides a buffer between the ambient atmosphere and the colder inner zone of the coolant stream.
Although such a heating element improves the distance over which laminar flow of the coolant stream can be achieved, there remain undesirable effects which limit its utility. The addition of an extension to the nozzle introduces a discontinuity in the flow containment which can disrupt the laminar nature of the coolant stream flow. Additionally, the heating elements used are omnidirectional, that is, they radiate heat outward into the ambient atmosphere as well as inward into the coolant stream. Because warming the outer zone of the coolant stream requires a significant amount of heat, the heating element presents a safety hazard to personnel working around the nozzle.
It is an object of this invention to provide a coolant stream nozzle with improved flow characteristics for the coolant stream after it exits the nozzle.
It is a further object of this invention to provide a coolant stream nozzle which allows greater separation distance between the nozzle and the sample being maintained in the coolant stream.
It is another object of this invention to provide a coolant stream nozzle which provides a coolant stream which limits icing of the sample being maintained in the coolant stream.
A coolant stream nozzle is provided which allows the outer zone of the coolant stream to be warmed inside the nozzle. The nozzle comprises a tubular cavity which guides the coolant stream within the nozzle, and an outlet from which the coolant stream exits the nozzle. The tubular cavity is enclosed in a vacuum jacket which insulates the tubular cavity, and thus the coolant stream, from the ambient atmosphere. Inside the vacuum jacket, an electrical heater is wound around the portion of the tubular cavity which is essentially adjacent the outlet. In the preferred embodiment, the outer surface of the tubular cavity comprises a scalloped heater seat which allows maximizes the physical contact between the electrical heater and the tubular cavity. The scalloped heater seat provides a continuous, threaded groove into which the heater is seated.
Also in the preferred embodiment, the electrical heater comprises an active heater wire and a heater lead wire, wherein the heater lead wire provides an electrical connection to the active heater wire, but does not itself produce significant heat. As those of skill in the art will recognize, it is desirable to have the heater as close to the end of the tubular cavity adjacent the orifice as possible, and also to have the heating effect on the coolant stream restricted to the linear length of the heater. Because the heater lead wire does not produce significant heat, it may be extended within the vacuum jacket to a convenient feed-through or connection point without producing undesired heating effects on the coolant stream.
An additional aspect of the preferred embodiment is to provide a flared portion of the tubular cavity essentially adjacent the outlet. It is preferred that the active heater wire is in contact with the tubular cavity for the length of the flared portion of the tubular cavity. Thus, the heater will provide heat to the coolant stream in a zone where the gas in the outer zone of the coolant stream will have room to expand as it is heated. Allowing this expansion to occur in the same area in which the heat is applied to the coolant stream and while the coolant stream is still enclosed by the flared portion of the tubular cavity helps to limit turbulence and maintains the laminar nature of the coolant stream flow.